Radio receiving system



Sept. 11, 1945.

J. R. DAVEY RADIO RECEIVING SYSTEM Filed May 23, 1944 3 Sheets-Sheet l Faun/250 FIG. 3

RAD/0 RECEIVER L 1m rm All LIMITEI? AME RAD/0 RECEIVER CH4 NNEL 4 POLAR/ZED A A A L RAD/O RECEIVER 309 AMP.

L lM/TER AMP.

CHANNEL 8 INVENTOR J. R. DA V5) A TTOPNE) W 1945- J. R. DAVEY RADIO RECEIVING SYSTEM 3 Sheets-Sheet 2 Filed May 23, 1944 0/? DA l E V N Q I/VVE'NT J. R.

wmm $3538 W kmN vwN ATTORNEY Sept. 11, 1945. J. R. DAVEY RADIO RECEIVING SYSTEM Filed May 23, 1944 3 Sheets-Sheet 5 w QWEEYIU INVENTOR J A. DAVE? 4 III! I I ATTORNEY Patented Sept. 11, 1945 RADIO RECEIVING SYSTEM James R. Davey, New York, N. Y., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application May 23, 1944, Serial No. 536,925

23 Claims.

This invention relates to radio telegraphreceiving circuits and more particularly to diversity receiving circuits for use in radio-telegraph systems employing diiierent frequencies for marking and spacing signals.

It is an object of the invention to provide a radio receiving system with improved means for detecting noise currents.

- It is also an object of the invention to provide a radio receiving system with improved means for preventing excessive noise currents from causing errors in the recording of signals.

It is also an object of the invention to minimize the objectionable effects of fading and to increase the reliability of reception in a diversity radio receiving system.

. An additional object of the invention is to provide a diversity radio receiving system with improved means for selecting among its diversity channels that channel which has the best instantaneous signaling conditions.

Another object is to provide a diversity radio receiving system with improved means for separately detecting noise currents in each of the diversity channels. I

A further object is to provide a diversity radio receiving system with improved means for decreasing the gain of a channel having poor signaling conditions while increasing the gain of a channel having good signaling conditions.

Another object is to supply a diversity radio receiving system with means for effectively disconnecting one of its diversity receiving channels whenever the difierential between the currents in the marking and spacing paths in that channel becomes low due to poor signaling conditions.

Still another object is to supply the receiving relay of a diversity radio receiving system with approximately constant current from the signal input circuits in spite of variations in signaling conditions in the individual inputcrcuits.

These and other objects are accomplshed by means outlined in the following brief description of the invention. In a radio receiving system for receiving, for example, marking and spacing radio-telegraph signals having different frequencies, the received currents of the two frequencies are first subjected to amplitude limitation and are then selectively amplified in separate amplifiers. An electronic tube is connected to the center tap of a large resistance bridged between the control grids of the marking and spacing output amplifier tubes for detecting average noise currents. Since the amplitude limitation is performed at a point in the circuit ahead-of the resistance, a change in the amplitude of the noise currents represents by itself a change in the signal-to-noise ratio. When a poor signal-to-noise ratio exists, such as when the signal is weak or absent, the differential between the currents in the marking and spacing paths becomes low thereby increasing the voltage at the center tap of the resistance. This causes an increase in the plate current of the electronic tube which, in one embodiment of the invention, is utilized for energizing a cut-off relay to remove the voltage normally applied to the screen grids of the marking and spacing output amplifier tubes. This method of noise detection is applied to a diversity radio receiving system by connecting a similar center tapped resistance with corresponding tube and relay circuits in each diversity channel. The cutofi relays in these diversity channels are energized differentially to out off only the noisiest channel thereby avoiding the possibility of disabling all the diversity channels at the same time.

' In accordance with a modification of the inventionythe receiving relay of a diversity radio receiving system is supplied with approximately constant current from the diversity signal input circuits by connecting'the output of the electronic tube, described above, in each'diversity channel to a series of amplifying tubes. The output of these amplifying tubes in each channel is applied to reduce the screen grid voltage of the output amplifier tubes in'that channel and to increase the screen grid voltage of the output amplfier tubes in the other channel. Thus, noise currents in one channel will cause a reduction in the gain of that channel and a corresponding increase in the'gain of the other channel so that the value of the combined currents supplied by both channels to the receiving relay remains approximately constant.

Another modification of the invention for supplying constant current to the receiving relay of a diversity radio receiving system, even under conditions where fading may occur in one channel during an absence of noise, comprises employing a common series resistance in the plate supply circuit for each output amplifier tube and in the screen grid supply circuit for the corresponding output amplifier tube in the other channel. This compensates for a weakening of the strengthof the signals in one channel by causing the receiving relay'to be provided with approxi mately constant current whether one or both of the channels is receiving signals because a decrease in the plate current of one tube will efiect a proportional increase in the screen grid voltage of the corresponding tube in the other channel which will in turn effect a corresponding reduction in the screen grid voltage of the first tube. Thus a weakening of the signal strength in one channel causes a reduction in the gain of that channel and a proportional increase in the gain of the other channel independently of the signal-tonoise ratio in either channel.

These and othel ieeiures of th invention are more fully described in connection with the fol-" lowing detailed description of the drawings in which: i

Fig. 1 shows a radio receiving system for receiving marking and spacing radio-telegraph signals and having means for detecting thev average noise currents present in the marking and spacing paths of the system;

Fig. 2 represents a dual diversity radio receiving system having means for reducing the gain of a noisy channel while correspondingly increasing the gain of the other channel in order to provide the receiving relay of the system with approxi.- mately constant current during the time that signals are being received;

Fig. 3 represents, a modification of the invention for supplying the rebfii ling relay of a diversity radio receiving system with. nearly constant currentfrom the diversity signal input circuits in spite of variations in the signal-to-noise. ratio; and I Fig. 4 illustrates av dual diversity radio receiving system provided with the noise detecting oilcuit of Fig. l and the constant current receiving relay circuit of Fig. 3. I

In Fig. i the signal input. to a radio-telegraph receiving circuit is represented by an antennaA connected to a radio receiver RR. which is com nected by a conductor l,v to a. current limiter CL, The output of the. current limiter CL is connected to two. parallel band-pass filters 2 and 3. which perform the function of separating, the marking and spacing telegraph, signals. In this embodiment oi the invention the marking signals have a frequency of 2125 cycles. and pass through filter 2, whil the spacing signals have av frequency of 29!?5 cycles and pass through filter 3 Following filter 2, the markin signals pass. through an amplifier 4 which may be of any con-. ventional design, such as. a thermionic tube. Amplifier 4 has its output coupled by a, transformer- 5 to a, detector 6 which is represented as. being a. full-wave coppe -oxide rectifie The. rectifier curre t hi h has ave har s mi r to t .e vel pc o the altern t n u r n enal appears, across the resistance to and isinpressedon the control grid of the. marking output. thermionic amplifier l where it is. amplified. The output of amplifier i passesfrom the. plate of the tube 1 through the upper winding of thepolarized; receiving relay & in the teletypewriter station equipment 2-3. This causes the relay c to move; its armature to its marking contact for controlling; othe ppara us t shown n the tas -P writer station equipment 23 to efiect. the record, ing of a, marking; signal. a

Simila y, pa ing s gnals ravel from er t through; an amplifier 9. and hr ug he trans.- former in to, th spacing; detector H The. rectified spacing ignals appear across the resistance 1% and; are suppliedtothe control grid of the spacingoutput the mio c am fier 1 w i h has it output conn ted tom o er in ns of h receiving relay 8 for causing th armature of. ro l v 8 t move to it ci g contfl9 fi 0 recording; of aspacing signal. a

A relatively larg resistance in the'form of two equal series-connected resistances I3 and I4 is bridged across the control grids of the marking and spacing output amplifier tubes 1 and I2. The instantaneous value of the voltage at the midpoint I! between the resistances l3 and M will be the average of the voltages impressed at that particular instant upon the grids of the marking and spacing output amplifier tubes 1 and 12. A center tap 15 connects the mid-point I 1 between the resistances l3 and M with an electronic tube It, which-in this particular form of the invention is a triode. As the amplitude limitation by the current limiter CL is performed at a point in the circuit ahead of the detectors 6 and l I, a change in the amplitude 01 the nois represents, by itself. a change in the signal-to-noise ratio. Thus the tube. it} serves to detect the instantaneous value of the average of the noise currents in the marking and spacing paths.

For example. when a marking signal wellabove the noise level isreceived. a voltage will be applied to the control grid of the marking amplifier I. This voltage will be equal to the cathode potential which i held at some positive potential. As no. rectified voltage will appear at this time across the resistance 2!, th control grid of the spacing amplifier l2 will be at ground potential and its plate current will be out off. However. the plate current of the marking amplifier I will be of substantial value and will so energize the receiving relay 8 as to cause it to operate its armature to its marking contact. Under these. conditions, the voltage at the midpoint I! will be one-half of the value of the volt age supplied to the control grid of tube 1-.. Similarly, when a spacing signal well above the noise level is received. the voltage conditions described above become interchanged between tube 1 and tube I: with the result that the voltage at the mid-point I! will now have a value equal to onehalf the voltage of the rectified signal applied to. the control grid of the tube l-2. As these markingand spacing signals have been assumed.- to be of full strength and well above the noise level. the voltage at the mid-poi nt t1, remains at the same value as long 8d this; condition exists regardless of whether the signals; are marking or spacing.

However, when transmission and other; conditions are such that. a poor signal-to-noise ratio exists, such. as when the signal is weak or. absent. and thenoise currentsor other interferin waves have frequency values corresponding to the signaling frequencies so that they are. passed through filters 2 and 3:, the differential. between the currents in the marking and spacing paths becomes lowthereby increasing thevoltage at the mid-point l1. When a marking signal is re-' eive nder these conditions. he. output or the p c detector l-l. will not be zero. butwill be some positive value depending upon the value. of the noise currents.- passed by the spacing filter 3. The voltage at th mid-point. I! will now not be merelyone-halfi that. applied to the control grid of tube 1, but. will'havo a. larger value which is. equal to, theaverage: of the rectified signal current app ied t the. control grid of. tube 1' and the rectified noise currents. applied to. the control grid of tube 12. A similar condition will exist.

when a. spacing signal is received. Thus under these condition the voltage at the mid-point I] will be; increased to alargervalue than itwas when such noise, currentswere absent.

As. this increase in, the value of the voltage at the mid-point l1 follows closely therise in telegraph'distortion caused bynoise currents, it provides a means for detecting the existence of high distortion conditions due to poor mark-spacediscrimination. -'It also provides means for detecting the instantaneous value of the average of the noise currents in the marking and spacing paths of a signaling channel. The detecting function of tube i8 is not directly responsive to the signalto-noise ratio, since an increase in the voltage at the mid-point l1 represents by itself a change inthesignal-to-noise ratio.

This voltage increase can be utilized to perform various useful functions. For example, it can be applied to the grid of the electronic tube 18 to effect; a proporti'onal increase in its plate current. The plate circuit oftu be l8 includes the operating winding of a polarized cut-off relay 19 which has its biasing winding supplied with biasing-current from a battery 20. This biasing current tends to hold the armature of relay l9 against its contact as is shown in Fig. 1. Normally, the plate current oftube I8 is not sufiicientlylarge to overcome the biasing current. However, when a poor signal-to-noise ratio exists, thediiferential between the currents in the marking and spacing paths becomes low due to the presence of rectified noise currents and the value of the voltage at the mid-point i'l becomes increased thereby effecting a proportional increase in the plate current of tube 18. When this plate current, becomes large enough to overcome the effect of the biasing current, the cut off relay l9 will operate its armature away'from its contact. This opens the path frombattery 22 to the screen grids of the marking and spacing output amplifier tubes 1 and I2 so that they will remain cut off during the time that excessive noise currents are present.

When the strength of the noise currents diminishes, the valued the voltage at the mid-point I! will be lowered and will effect a proportional decrease in the plate current of tube 18. As soon as the plate current becomes sufiiciently low, the biasing current from battery 20 will again operate the armature of relay J9 to its contact. This closes the path from battery 22 to the screen grids of tubes 1 and i2 to enable .these tubes to resume their amplifying functions; Thus thedeleterious efiects, such as errors inthe recording of signals, of selective fading or excessive noise currents are avoided.

"In Fig.2 the principles of the invention that were described above for detecting the existence of high distortion conditions in a radio telegraph receiving circuit are shown applied to a dual -diversity radio receiving system for decreasing the gain of the signaling channel having the poorer signal-to-noise ratio while increasing the gain of the other signaling channel. This system is shown to be provided with two separate radio receivers 20l and 202 connected to their respective antennas 203 and 204 for receiving radio-telegraph signals from the same transmitting station.

'The'receiving' antennas 203 and 204 are installed at different locations for minimizing the effects of selective fading in accordance with the principles of space diversity reception. The advantage of space diversity reception lies in the fact that a fading out of the signals does not usually occur at both receiving antennas simultaneously. This invention enhances the benefits derived from spa ce diversity reception by preventing the energy in a channel having a 'po'or' signal t'o no'ise ratio "due to a faded signaland havingan output consisting chiefiy ofnoise from impairing the reception of signals by the other channel. Therefore, one of the objects of the circuit shown in Fig. 2 is to use the noise voltage detected in each channel for reducing'the gainin that channel. In other words, if signals in one of the channelsare decreasedinamplitude due tofading, that channel may receive'considerablymore noise or static energy-thanthe otherchannel; When this occurs, the noise sup pression circuits shown in Fig. 2 will reduceth'e strength of the signals supplied by the-noisier channel-to' the windings of the receiving relay 236 and, at the same time,'-will increase the strength of' the signals supplied bythe quieterchannell. J 1- The principal utility of the circuit shown in Fig. 2 lies in'protecting' the system against noise in the faded outchannel; that is, the noise suppression circuits function when the signals in one of the channels are very weak or have disap peared.- If the signals were strong, the current limiter would suppress'most of the *nois'e'and no other protection would be necessary. If the sig-' nals were absent and no noise were present, no protection would be needed. Therefore the noise suppression circuits are of greatest utility when the signals are weak or absent and the noise isstron'g; W

It can be seen that the first portion of the circuit of-Fig. 2 is similar to that of Fig;" lin that the output of the radio-receiver -20l in channel A-is connected to a current limiter 205 which has its output connected to the parallel bandpass'filters 206 and 201.- Filter 206 passes the marking signals having a frequency of 2125 cycles 'and'filter 201 passes the spacing signals having a frequency of 2975 cycles. The marking filter 206 has its output connected to an ampli-' fier 208 and a rectifier 209 while the spacing filter 201 has its output connected to a similar amplifier 2I0 and rectifier 2. The output of the marking rectifier 209 supplies the rectified signals to the controlgrid of the marking output amplifier-tube 212 and the output of the spacing rectifier 2H supplies the control grid of the spacing output amplifier tube 2I3.' A center tapped resistance consisting of two equal series resistances 2E4 and 2|5 is bridged across the control grids of the amplifier tubes 212 and 2I3. A point 2| 6 which is electrically one-halfv way between the control grids of tubes 2l2 jand 2|3 forms a junction between the resistances 2| 4 and 215. A conductor 2| 1, which is the center tap of the 'resistanceconstituted by the resistance elements 2 l4 and 2 l 5, connects the junction' point 2|6 to the control grids of twoelectronic tubes 218' and 2i9- in the noise suppression circuit NSCI.

In channel B, the radio receiver 202 has its output connected to a current limiter 240. It should be noted that the outputs of the radio receivers 20| and 202 are also connected to an automatic frequency control circuit 260. The current limiter 240 in channel B has its output connected to the parallel marking and spacing filters 24! and 242. These filters supply the am plifiers 243 and 244 and the detector 245 and 246 which, in turn, supply the rectified signals to the control grids of the marking and spacing output amplifier tubes 232 and 233. A resistor consisting'of the two equal series resistances 241 and 248 is bridged across the control grids of tubes 222 and 233. The mid-point; 249 between the resistances 241 and 248 is connected by conductor 250 to the. control grids of two electronic tubes 25 and 252 in the noise suppression circuit N802.

As was described above in connection with the description of the operation of Fig. 1-, the instantaneous value of the voltage at the junction point 216 will be the average of the voltages sup-- plied. to the control grids of tubes2 i2 and'2 l3and, likewise, the instantaneous value of the voltage at the mid-point 249 will be the average of the voltages supplied to the control grids of tubes 232 and 233. v 7

.When conditions are such that a poor signalto-noise ratio exists in either the marking or spacing paths of either channel A or B, the voltage at the mid-point in such channel will rise to a more positive value. As this rise in voltage follows closely the rise in telegraph distortion caused by the noise, it provides a. means for detecting when high distortion conditions exist due to poor mark-space discrimination. This rising voltage i utilized by the noise suppression circuits NSC! and NSC2 to reduce the rectified currents-from.- achannel when the noise in its output becomes excessive and to correspondingly increase the rectified currents from the other channel. During operation of the system,.the rectified signals from each channel are combined so that the combined output f the marking output amplifler tubes H2 and 232 is supplied over conductor 235 to the upper winding of the polarizedv receivingv relay 236 and, a1ternatively,.the combined output of the spacing output amplifier tubes 21-3 and 233 is supplied overconductor 231 to the lower winding of the relay 236.v In the case of a marking signal, relay 236 willoperate its armature to its left contact to connect. the negative pole of battery 238 over the armature. of relay 236, through resistance. 254, winding of the polarized receiving relay 255 in the teletypewritcr subscribers set 256, and then. along conductor 25?! to the mid-point of a potentiometer comprisingthe resistances 258 and 259. Similarly, in: the case of a spacing, signal, relay 236 will operate its armature to its right contact. to connect. the positive pole of battery 238 over a corresponding path through the winding of. the receiving relay 255 to the mid-point of the potentiometer com-- prising. the resistances 258 and 259.

It can be seen from this that marking. signals will cause current from, battery 23! to flow through the circuit to the receiving relay 255: in one direction and that spacingsignals willcause an equal current to flow in theoppositedirection. Thus the transmission of signals to the receiving relay 255 is orr a polar signaling. basis. If current from a rectifier is used instead of. current from battery 238,. the above-described: circuit' will permit the polar transmission of signals to the receiving relay 255' to be practically independent of. ordinary variations in the rectifier voltage. V I

Considering, now the. noise suppression circuit. N801. the plate current. for tube 2l-8 flows from battery 224 and. through a resistance 221%. which is also the seriesscreenresistance for tubes H2; and 213', and then along a conductor 2202 to the plate oi v tube 2l 8.. Tube 2-l-8 isnormally biased to near cut-01L. Ii channel-.A becomes noisysome of the. noise currents will rectified in the: spacing detector ill I. while. marking signals 'are' being rectified. in the marking detector 2091 and vice 61:53 As. was stated above, thiswill raise the voltage at. the junction point 2% and this in.-

creased. voltage will be supplied; over conductor 2H. to the control grids of tubes 2 and 2" to effect a proportional increase in the plate cur rents of these tubes. The increased plate current of tube 2 will flow over conductor 220 mm through the resistance 221 andwill thereby cause an added voltage drop through the resistance 22l-. Since resistance 22! is a series screen resistance for the marking and. spacing output amplifier tubes 2E2 and. M3, the screen voltage and plate currents of these-tubes will be correspondingly reduced thereby producing a proportional reductionin the gain: of channel A with a corresponding reduction. in the output supplied. fromthis channel to the receiving relay 236; Thus the noise components in channel. A aresuppressed in amplitude by an amount proportional to theamount of noise.

The increased voltage at the junction point 216. is also supplied to the control grid. of. tube 219 for proportionally increasing its plate current which flows through the resistance 223 to battery 224. In order. toobtain a tum-over in phase relation, tube 2l-9 is coupled to tube 225m such a manner as to produce an. opposite eftectin tube 225. Accordingly, tube 225,. which normally has a plate current near maximum value,. will have a decreasing plate current as the voltage at point 2IG increases. This coupling. is eflectedby a potentiometer consisting of two series resistances 226 and 221, resistance 226 being. connected to the plate of tube 2I5 and resistance 22!- being. connected to ground 228. The mid-point 220 between resistances 226 and 221 is connected to the control gridof tube 225.

The potentiometer consisting of resistances 225 and 221 is designed to maintainthepotential-on the control grid of tube 225 at one-third-ot that on the plate of tube 219. Thus when the plate current of tube 219 flowing through resistance 223 increases due to the presence of noise in channel A, the plate voltage of tube 213 will decrease and the grid of tube 225 will become less positive thereby effecting, a corresponding decrease in the plate current. of tube 225. The circuit elements are so designed that the sumoi the plate currents of tubes 2 l8 and 225 remains practically constant.

Plate current for tube 225 passes. alonglconduct0r230' and through the resistance23t tobattery 253. Resistance 23! is the seriesscreenrcsistance for the marking and spacing output amplifier tubes 232 and233'in channelB. Therefore, when the plate current of tube 225=decreases,.the

. voltage supplied to the screen, grids of tubes 232 and 233 increases proportionally and causes the current supplied by tubes. 232 and233-to'the. receiving relay 236 tobe correspondingly increased; In other words, the current supplied to the receiving relay 236 by the noisy channe1A.is.'reduced by a certain amount whilethe current supplied by channel B. is increased by the same amount so-that the result is that the combined current supplied to the receiving relay 235' by both channels A and'Bis maintained at anearly constant value. This suppression of the noise currents in channel A also producesa reduction in the amount'oftelegraph' distortion due to the fact that since theproportion of current supplied by the noisy=channel A is lowered below the normal one+halt condition, its-noise components are not able-to:cause any appreciabl'e'amount'of distortion.

Thenoise suppressionciicuit-NSCZ in channel B issimilar: in' construction to'the-noise'suppres anew sion circuit NSC! in channel Aand its operation and function are the same as explained above in connection with the description of the noise suppression circuit NSCI.

If both channels A and B are equally noisy, the potentials of the junction points zlB and 249 are raised equal amounts so that although the plate currents in tubes 2l8 and 225 in the noise suppression circuit- NSCI in channel A and the plate. currents in tubes 25l and 234 in the noise suppression circuit NSCZ in: channel B will change, this change will be symmetrical. There'- fore, both the sum of the plate currents of tubes H8 and 234 and the sum of the plate currents of tubes 25] and 225 will remain constant. Consequently, as no voltage change will occur on the screen grids of the marking and spacing output amplifier tubes in either channel A or 3,, no suppression will be effected at this time.

It is to be understood that the forms of the invention shown and described herein are preferred embodiments disclosed for purposes of explaining the nature of the invention. The invention is not to be limited to these specific forms as it may be applied to other systems, such as interrupted tone or interrupted continuous wave systems provided such systems had two current pathsbetween which a comparison could be made of the current values.

Fading may sometimes occur in some diversity receiving systems on occasions when there may be little noise eiiects and the net result of such selective fading may be to cause the absence of marking or spacing currents in one of the channels for someperiod of time. spacing currents fade out of one channel for a period of time and if the receiving relay in the teletypewriter station equipment is supplied by is rounded considerably, this two-to-one current.

condition will cause considerable bias of the signals and increase the distortion.

To avoid errors in the recording of signals which might otherwise be caused by such selective fading, the circuit shown in Fig. 31s, designed to supply constant current to thereceiving relay 365 in the teletypewriter station equipment 3l6 even. under conditions where fading may occur in one channel during the absence of noise currents.

In the circuit of Fig. 3, signal current of reduced strength in one channel effects a decrease in the A gain of that channel and a corresponding increase in the gain of the other channel. Although the constant current circuit is shown-applied to a dual diversity receiving system for receiving ra-' die-telegraph signals employing waves of i different frequencies, for markingand spacing signals, the circuit is not restricted to merely this type of signaling system but may be. applied to various other types of diversity receiving systems such as interrupted tone or interrupted continuous wave systems.

The system shown in Fig. 3 comprises two channels A and Beach havinga radio receiver 30! and 332 for receiving signals from the same transmitting station over the separate antennas 303 and 304 which are located at different points in accordance with space diversity principles. Each radio receiver has its output connected to a current limiter 305 and 306. The output of each If themarking or current limiter is connected in parallel to a set of marking andspacing filters 301 and 308 in channel A and 309and 3|.0 in channel B.. The output of each of these filters is separately amplified and rectified in separate amplifiers and detectors as was described above in connection with the description of the operation of Figs..1 and 2. The resulting rectified signal currents are supplied to the control grids of the marking and spacing output amplifier tubes in each channel,

tubes 3]! and 1-H: being located in channel A and tubes 3|3 and 3 it being located in channelB. In order to supply nearly constant current to thepolarized receiving relay 3l5 in the teletypewriter station .equipment3l6 regardless of differences in the strength of thesignal currents in channels A and B, the plate of each amplifier tube is supplied from a. battery and through 'a resistance which is common to' and in series with the supply circuit for the screen grid of the corresponding output amplifier tube in the other channel. Specifically, the marking output amplifier tube'3l I in channel A has its plate connected by the conductor 33l through the resistance 311 to the battery 330 and the supply circuit for the screen grid of the marking output amplifier tube 313 in channelB extends along the conductor 318 through the common series resistance 3|! to the battery 33!). The screen grid supplycircuit for, tube 3 extends along conductor 3H! and through the resistance 320 to the battery 33!) and the plate of the marking output amplifier tube 3l3 in channel B is connected by conductor 332 through the common series resistance 320 to battery 333. The spacingoutput amplifier tubes 3 i 2and 3 l 4 in channelsA and B have their plates and screen grids similarly supplied from battery 338 through the common series; resistances 32! and 322. 4

By thus employing common series resistances in the plate and screen .grid supply circuits for the marking and spacing output amplifier tubes in channels A and B, the receiving relay 3 !5 will be provided with approximately constant current. This is true whether one or both of the channels is receiving signals because a decrease in theplate current of one output amplifier tube in one channel will increase the screen grid voltage on the corresponding output amplifier tube in the'other channel which will in turn reduce the screen grid voltage on the first tube. In other words, a weakening of the signal strength in one channel causes a reduction in the gain of that channel and a corresponding proportional increase in the gain of the. other channel independently of the signalto-noise ratio.

For example, a decrease in plate current of the I marking output amplifier tube 3 in channel A causes a higher screen. grid voltage to be applied to the marking output amplifier tube 3l3 in channel B. This causes a proportional increase in the plate current of tube 3 I3 which in turn, by

, means of the above-described employment of. the

commonseries resistances 311 and .32 0 reduces by a proportional amount the screen grid voltage applied to the marking output amplifier tube 3| I in channel A.. The value of the common series resistances is not made high enoughto cause instability in the normal approximate equal division of the relay currentsupplied by the two tubes. The result, however, is that the relay v current remains approximately the same whether by the fading out of either the marking or spacing current in one channel is reduced to a small amount. This constant current circuit finds its greatest utility in a system using narrow band'- pass filters where the signals are rounded to such an extent that large amounts of bias would otherwise result.

It should be noted that there is a distinction between the method of operation of the circuit of Fig. 3 and the circuit of Fig. 2 described above. This distinction resides in the fact that in the circuit of Fig. 2 the changes in the gain of the signaling channels is responsive to changes in the strength of the noise currents present in the channels, whereas in the circuit of Fig. 3 similar changes in the'gain of the channels are effected in response to changes in the strength of the signaling currents present in these channels. In neither of these circuits is the gain control action responsive directly to the signal-to-noise ratio.

Fig. 4 shows a dual diversity radio-telegraph system employing different frequencies for marking and spacing signals and which combines the inventive features of the noise detecting circuit shown in Fig. 1 with the constant current receiving relay circuit of Fig. 3. The circuit elements function in the same manner as described above in connection with the descriptions of the operation of the circuits of Figs. 1 and 3. The system includes two antennas 40! and 402 located at different points for receiving signals from the same radio transmitter. These signals are supplied separately by the antennas 40l and 402 to the radio receivers 403 and 404 which are separately connected to the current limiters 405 and 406. The outputof each limiter is connected to two parallel band-pass filters 401, 408, 409 and 410 for separating the marking and spacing signals.

Each filter has its output connected to one of the amplifiers 4, M2, M3 and 404 which are provided with automatic volume control means of any suitable design including theconductors M5 in the upper channel A and the conductors M6 in the lower channel B. The outputs of the amplifiers are rectified in the detectors 4H, M8, M9 and 420 and the rectified signal currents are then supplied to the control grids of their respective output amplifier tubes 42l, 422, 423 and 424. The plates of the marking amplifier tubes 42l and '423 in channels A and B are connected together so that their combined output is applied to the upper winding of the receiving relay 425 in the teletypewriter station equipment 426. Similarly, the plates of the spacing amplifier tubes 422 and 424 are connected together scthat their combined output is applied to the lower winding of the receiving relay 425.

A center tapped resistance comprising the equal series resistors 42'! and 428 is bridged across the control grids of tubes 42! and 422 in channel A and has its mid-point connected to the control grid of a triode 429. Likewise, a similar center tapped resistance comprising the equal series resistors 430 and 43] is bridged across the control grids of ,tubes 423 and 424 in channel B and has its midpoint connectedto the control grid of a triode 432. The plate circuit of tube 429includes the operating Winding of the cut-off relay 434, the nonoperating Winding of the cut-off relay 435 and the battery 433. The plate circuit of tube 432 ineludes the operating winding of relay 435, the non-operating winding of relay 434, and thebattery 433. The holding or biasing winding of both relays 434 and 435 is supplied with biasing current from battery 45] for holding its armature against its contact when the currents from tubes 42! and 432 are equal.

As was explained above in connection with the description of the operation of the circuit shown in Fig. 1, the instantaneous value of the voltage at the mid-point between the resistances 421 and 428 will be the average of the voltages impressed at that particular instant upon the grids of the marking and spacing output amplifier tubes 42! and 422 in channel A. The result is that an ex cessive amount of noise imchannel A will increase the plate current of tube 423. If the increased plate current of tube 423 is'greater than the combined value of the currrents in the biasing and non-operating windings of relay 434, then relay 434 will operate its armatures thereby opening the paths leading from battery 450 to the screen grids of tubes 42l and 422 so that these tubes will remain cut of! during the time that excessive noise currents are present in channel A. Similarly, if excessive noise currents in channel B cause the plate current of tube 432 to become greater than the combined value of the currents in the biasing and non-operating windings of relay 435, then relay 435 will operate its armatures thereby opening the paths extending from battery 450 to the screen grids of tubes 423 and 424 to cut off these tubes during the time that excessive noise currents are present in channel .8. This method of differentially operating relays 434 and 435 avoids the possibility of cutting both channels oil at the same time and insures that at least one channel will always be operative and that only the channel having the poorer noise condition will be disabled or out 01f. The presence of biasing current in the biasing windings of relays 434 and 435 prevents either of these relays from operating unless the difference between the plate currents of tubes 423 and 432 is greater than the value of the biasing current. If this difference is less than the value of the biasing current, both channels A and B will remain connected to the receiving relay 425. Likewise, if the plate currents of tubes 429 and 432 are equal, their difierence will be zero and the armatures of relays 434 and 435 will remainin the condition shown in Fig. 4.

To provide the receiving relay 425 with approximately constant current regardless of differences in the strength of the signal currents in channels A and B which may occur because of selective fading during an absence of noise, as explained above in connection with the description of the operation of Fig. 3, the plate of the marking output amplifier tube 421 in channel A is connected through the resistance 438 to battery 450 and the supply circuit for the screen grid of the marking output amplifier tube 423 in channel B extends along the conductor 439 and through the common series resistance 438 to battery-450. The screen grid supply circuit for tube 42| extends along conductor 440 and through the resistance 44! to battery 450 and the plate of tube 423 in channel B is connected through the common series resistance 44! to battery 450. The spacing output amplifier tubes 422 and 424 in channels A and B have their plates and screen grids similarly supplied from battery 450 through the common series resistances 442 and 443.

By thus employing common series resistances in the plate and screen grid supply circuits for the marking and spacing output amplifier tubes in channels A and B, as was explained above in connection with the description of the operation of the circuit shown in Fig. 3, a decrease in the plate current of one output amplifier tube in one channel will increase the screen grid voltage on'the corresponding output amplifier tube in the other channel which will in turn reduce the screen grid voltage on the first tube. Thus a weakening of the signal strength in one channel causes a reduction in the gain of that channel and a corresponding proportional increase in'the gain of the other channel. This is accomplished independently of the signal-to-noise ratio. The result is that the combined current supplied from channels A and B to the receiving relay 425 remains approximately the same whether both channels are receiving signals or whether only one is receiving signals.

What is claimed is:

1. A radio-telegraph receiving system adapted to receive marking and spacing signals having different frequencies and including in combination a current limiter, a first band-pass filter for passing only waves of the marking frequency, a second band-pass filter for passing only waves of the spacing frequency, means for connecting the output of the current limiter to said'filters in parallel, a marking output amplifier tube havin a control gr d, a spacing output amplifier tube having a control grid, a marking path extending from the output of the marking filter to the control grid of the marking output amplifier tube, a spacing path extending from the output of the spacing filter to the control grid of the spacing output amplifier tube, and detecting means for detecting the instantaneous value of the average of the noise currents in the marking and spacing paths, said detecting means comprising two equal series-connected resistances bridged between the control grids of the marking and spacing output amplifier tubes, an electronic tube having atleast one electrode, and circuit means'for connecting the mid-point between said resistances to an electrode of said electronic tube. I

2. A radio-telegraph receiving system adapted to receive marking and spacing signals having different frequencies and including in combinaton a current limiter, a first band-pass filter for passing only waves of the marking frequency, a second band-pass filter for passing only waves of the spacing frequency, means for connecting the output of the current limiter to said filters in parallel, a marking output amplifier tube having a control grid and at least one other electrode, a spacing output amplifier tube having a control grid and at least one other electrode, each of sa d other electrodes having a supply circuit, a marking path extending from'the output of the marking filter to the control grid of the marking output amplifier tube, a spacing path extending from the output of the spacing filter to the control grid of the spacing output amplifier tube, and control means for preventing excessive noise currents from causing errors in the recording of signals, said control means comprising two equal series-connected resistances bridged between the control grids of the marking and spacing output amplifier tubes, an electronic tube having at least two electrodes, c rcuit means for connecting the mid-point between the resistances to one electrode of said electronic tube, an instrumentality for alternatively opening and closing-said supply circuits to said electrodes of the marking and spacing output ampl fier tubes, operat ng means for operating said instrumentality in accordance with current conditions in said electronic tube, and circuit means for electrically connecting said operating meansto the other electrode of said electronictube; .7

3. A diversity radio receiving system including at least two' signal receiving-channels, each of said channels'having detecting means for separately detecting noise currents, said detectin plurality of centertapped resistances each bein connected across a different channel, electroresponslve means in each channel adapted to'effectively disable'at least one of said channels, and connecting means for connecting the center tap of each of said resistances to a difierent one of said electroresponsive means. l

I 5. A diversity radio receiving system including at least" two signal receiving channels, a plurality of current limiters, eachof said cur'rent'limiters being connected into a different channel, detecting means for separately detecting noise currents in each channel, said detecting means comprising a plurality of electroresponsive means, a

center tapped resistance bridged across each' channel at a 'point subsequent 'to the current limiter in each channel, connecting means for connecting the center tap of each resistance to a different one of said electroresponsive' means, disabling means for selectively disabling said channels individually, and operating means for 'operating the disabling-means to disable the channel having the most noise currents at a particular instant as determined by the detection of noise currents by said detecting'means.

6. A radio-telegraph receiving system' for receiving" marking' and spacing I telegraph signals having diifer'ent frequencies, filter means for separating the marking and spacingsignals, first rectifying means for separately rectifying the marking signals, second'rectifyingmeansfor separa't'ely rectifying the spacing signals. and detecting means for detecting when'the differential between the-rectified currents from said rectifiers becomes low, said detecting means comprising a center'tapped resistance bridged between the outputs of both of said rectifiers.

I 7. A'radio-telegraph'receiving system for receiving marking and'spacing telegraph signals having difierent frequencies, filter means for separating the marking and spacing signals, first amplifier tube having an electrode connected to the output of the first rectifying means, a second amplifying tube having an electrode connected to the output of the second rectifying means, a

center tappedresistance bridged between said electrodes, detecting means connected to the .cen-

ter tap of said resistance for detecting when the differential between therectified currents from said rectifying means becomesilow, control means adapted to render said amplifier tubes incapable of producing anode-cathode current flow, and

governing means for governing the operation of said control means in accordance with the detection performed by said detecting means;

8. A diversity radio-telegraph receiving system for receiving marking and spacing telegraph signals having different frequencies, said system including at least two diversity receiving circuits each comprising filter means for separating the marking and spacing signals, first rectifying means in each circuit for separately rectifying marking signals, second rectifying means in each circuit for separately rectifying spacing signals, a marking amplifier tube in each circuit connected to said first rectifying means, a spacing amplifier tube in each circuit connected to said second rectifying means, each of said tubes hav-'- ing a screen grid and an anode, a receiving relay for effecting the recording of said signals, said relay having at least two windings, first connecting means for jointly connecting the anodes of the marking amplifier tubes of both circuits to one winding of said receiving relay, second connecting means for jointly connecting the anodes of the spacing amplifier tubes of both circuits to another winding of said receiving relay, and control means for supplying constant current to the Y windings of the receiving relay for avoiding deleterious effects caused by selective fading of the telegraph signals, said control means including a plurality of resistances for separately coupling the anode of each amplifier tube to the screen i grid of the corresponding amplifier tube in the other diversity receiving circuit.

9. A diversity radio receiving system having at least two diversity receiving circuits for receiving signals, each of said diversity receiving circuits having detecting means for detecting noise currents in each circuit separately, and control means for reducing the gain in a noisy diversity receiving circuit and for correspondingly increasing the gain in another diversity receiving circuit in' response to the detection of noise currents by said detecting means, said control means including a plurality of electronic tubes connected in each diversity receiving circuit, each of said tubes having a plurality of electrodes, and coupling means for coupling an electrode of at least one tube in each circuit with an electrode of at least a one tube in the other diversity receiving circuit.

10. A diversity radio receiving system including at least two signal receiving channels, a plurality of current limiters, each of said current limiters being connected into a different channel, detecting means for separately detecting noise currents in each channel, said detecting mean including at least two equal seriesaconnected resistances bridged across each channel at a point subsequent to the current limiter in each channel, disabling means for selectively disabling said channels, and operating means for operating the disabling means to disable all but at least one of the channels in accordancewith the detection of noise currents by said detecting means.

11., A diversity radio receiving system, having a plurality of signal receiving channels, each of said channels having detecting means for deriving a voltage representative of the instantaneous value of excessive noise .currents present in that channel, cut-on means for cutting oil said channels individually, and differential operating means for comparing said voltages for selectively operating the cut-01f means in accordance'with the instantaneous noise conditions in the signal receiving channels.

'12. A diversity radio receiving system, having a plurality of signal receiving channels, each of said channels having detecting means for deriv ing a voltage representative of excessive noise currents present in that channel, cut-ofl means for cutting off said channels individually, said cut-01f means comprising a plurality of differential relays each having a plurality of windings, and operating means for separately applying said voltages to different windings of each of said relays for selectively operating said relays.

13. A dual diversity radio receiving system com,- prising two signal receiving channels, each of said channels having detecting means for deriving a voltage representative of the instantaneous value of excessive noise currents in that channel, cut- 011" means for cutting off said channels-alternatively said cut-off means comprising two differential relays each having a plurality of windings, and circuit means for differentially applying said voltages to two windings of each of said relays for operating said relays alternatively in accordance with the instantaneous noise conditions in the signal receiving channels.

14. A dual diversity radio receiving system comprising two signal receiving channels, each of said channels including at least one electronic amplifier, each of said amplifiers having at least one electrode, a current supply circuit extending to each of said electrodes, each of said channels having detecting means for deriving a voltage representative of the instantaneous value of excessive noise currents in that channel, disabling means for cutting ofi" said current supply circuit from said electrodes alternatively for disabling said amplifiers alternatively in accordance with the instantaneous noise conditions in the signal receiving channels, and operating means for differentially applying said voltages to the disabling means for operating said disabling means.

15. A diversity radio receiving system having a plurality of signal receiving channels, a receiving relay having an operating winding, means for combining the currents in said channels and for supplying said combined currents to the winding of the receiving relay, each of said channels having detecting means for deriving a voltage representative of the instantaneous value of excessive noise currents present in that channel, regulating means for regulating the currents in the channels for stabilizing their combined value at an approximately constant amount, and operating means for applying said voltages to the regulating means for operating said regulating means in accordance with the instantaneous noise conditions in the signal receiving channels.

16. A diversity radio receiving system comprising in combination, a plurality of signal receiving channels, each of said channels including at least one variable gain amplifier, a receiving relay having an operating winding, means for combining the currents in said channels and for supplying said combined currents to the winding of the receiving relay, each of said channels having detecting means for deriving a voltage representative of the instantaneous value of excessive noise currents present in that channel, control means for varying the gain of said amplifiers for stabilizing their combined output currents at an approximately constant amount, and operating means for applying said voltages to the control means for operating said control means in accordance with instantaneous noise conditions in the signal receiving channels.

17 A diversity radio receiving system comprisng in combination a plurality of signal receiving channels, each of said channels including-at least one variable gain amplifien; a receiving relay having an operating winaing,; means;ior combining the currents iii-said cna l elstand for supplying i c me edc r nts toxthe windin 01 n receivin re ay, each 9 s id.- nen vi detecting, mea s .e r vi s :13 vol a e rep tative or theinstantaneous value or noise currents present in that channel, control means ior reducing the gain o1';,tl 1e ;an 1pl n er 1n a .noisy channel and Ior, correspondingly increasing the gain or the amplifier -ln;.anotner channel 101 stabilizing their co nbined outputgcurrents at an approximatelyconstant amount, and operating means for applying said voltages to the control means for operating said control means in accordance with the instantaneous noise conditions in the signal receiving channels.

,18. A diversity radio receiving system comprising in combmatlon, a plurahty or signal receiving channels, each or said channels including at least one variable gain amplifier and detecting means i'or deriving a voltage representative or the mstantaneous value or excessive noise currents present in that channel, regulating means 101' reducing the gain of the amplifier in a channel in wmcn excessive noise currents are present and IUI (JOIIeSPOIIQmEly increasing the gain 01 Hlii ampliner in another or said channels, salo. regulating means comprising a pluranty or multielectrode electronic tubes, and operating means lor applying the voltage derived in one or said channels to an electrode 01 at least a first one or said tubes and Ior applying the voltage derlveu. in another or said channels to an electrode or at least a second one 01 said tubes Ior controlling the current now in said tubes in accordance with the instantaneous noise conditions in said channels.

19. A diversity radio receiving system comprising in combination a plurality of signal receiving channels, each of said channels including at least one variable gain multielectrode electronic ampliiier and detecting means for deriving a voltage representative of the instantaneous value of excessive noise current present in that channel, regulating means for reducing the gain of the amplifier in a channel in which excessive noise currents are present and for correspondingly increasing the gain of the amplifier in another of said channels, said regulating means comprising a plurality of multielectrode electronic tubes, operating means for applying the voltage derived in one of said channels to an electrode of at least a first one of said tubes and for applying the voltage derived in another of said channels to an electrode of at least a second one of said tubes for controlling the current flow in said tubes in accordance with the instantaneous noise conditions in said channels, and control means for connecting another electrode of said first one of said tubes to an electrode of one of said amplifiers and for connecting another electrode of said second one of said tubes to an electrode of another of said amplifiers.

20. A dual diversity radio telegraph receiving system comprising two signal receiving channels for receiving marking and spacing signals having different frequencies, each of said channels having a first band-pass filter for passing only waves of the marking frequency and a second bandpass filter for passing only waves of the spacin frequency, each of said channels also having a marking output amplifier tube with a control grid and a screen grid. and a spacing output amp ifi r tub With a cont elgrid a d? s en d. each of said channelsiurther comprising a marking path for connectingthev output oi' its first band-pass filter to the, control grid of its marliing output amplifier tube ;and,a spacing patn lor connecting the output of its secondhand-pass alter to the control grid of its spacing output amplifier tubes, a receiving relay having at least two windings, first supply means; for combining the-output currents of both 01' saidmai-King output amplifier tubes simultaneously and for supplying said combined currents to one winding of thereceiying relay, second supply means for combining the output currents of both of said spac ing output amplifier tubes simultaneously and for supplying said combined currents to another Winding of the receiving relay, each of said screen grids having a voltage supply circuit, regulating means for producing the gain of at least one 01' said output amplifier tubes in one of said channels and for proportionally increasing the gain or at least a corresponding one of said output amplifier tubes in the other channel for stabilizing the value of their combined instantaneous outputs supplied to said receiving relay at an approximately constant amount, said regulat means comprising control means for reducing the voltage supplied to the screen grid of at least one OI said output amplifier tubes in one of said channels and for increasing by a proportional amount the voltage supplied to the screen grid of at least a corresponding one of said output amplifier tubes in the other channel, said control means including resistors connected in said screen grid voltage supply circuits.

21. A diversity radio receiving system comprising at least two signal receiving channels, each 01 said channels including at least one output amplifier tube having a screen grid, each of said screen grids having a voltage supply circuit, regulating means for reducing the gain of at least one output amplifier tube in one channel and for correspondingly increasing by an equal amount the gain of at least one output amplifier tube in another channel, said regulating means comprising control means for decreasing the voltage supplied to the screen grid of said tube in said first channel and for correspondingly increasing by an equal amount the voltage supplied to the screen grid of said tube in said second channel, said control means including resistors connected in said screen grid voltage supply circuits.

22. A diversity radio receiving system comprising at least two signal receiving channels for receiving signals, each of said channels having at least one output amplifier tube for amplifying said signals, each of said tubes having an anode and a screen grid, each of said electrodes having a voltage supply circuit, regulating means for increasing the gain of at least one output amplifier tube in one channel and for correspondingly decreasing by an equal amount the gain of at least one output amplifier tube in another channel,

said regulating means comprising control means for increasing the voltage supplied to the screen grid of said output amplifier tube in said first channel and for correspondingly decreasing the voltage supplied to the screen grid of said output amplifier tube in said second channel, said control means including a first common series resistance connected in the voltage supply circuits for the anode of said first tube and for the screen grid of said second tube, and a second common series resistance connected in thevoltagesupply circuits for the anode cf sarid-eecond tube and for the screengrid of said.- firsb tube.=-

2-3. A diversity radio receiving system-comprising at leasttwo signev1-receiving= channelsfor receivingsignals of varying--streng-th; each of said channelshaving at 1'east-- one output amplifier tube for amplify-ingsaid 'si'g-nal's, each.- of said tubes having an anode and ascreengrid,

receiving relay with approximately mutant current, saidcompensating means-including a, mat, commonseriesmeslstance ccnnected in the voltage-supply circuits forkhe anodeof at leash-one output amplifie-r tube in one channel audio!- the screen g-rid at at. least one eubpubamplifiez tube in anotahei channel, and a second common series resistance ccnnectedin the voltage supply circu-its for the amede of said second {aube andaiorcach of said-electrodes having a vol-tagesupply' 10 the soreen grid ot'said firsbmube, said-receiving circuit, a receiving re1ay=-having operating: winding; compensating means for compensating for a weakeningin-the strength creme-signals in onechannei by supplying the winding 01* the relay having itsoperating winding cbnnecteto each of said voltage supply circuits.

JAMES R.-DAVEY. 

